1. Field of the Invention
This invention relates generally to telecommunications, and more particularly, to wireless communications.
2. Description of the Related Art
Growth of wireless communication systems has increased the demand for highly efficient amplifiers, for example, power amplifiers like radio frequency (RF) amplifiers. A power amplifier is an active, two-port device that exhibits both linear and non-linear behavior. Some design parameters for RF power amplifiers include high output power, high linearity, and good efficiency. The linearity of a power amplifier may refer to an ability of an amplifier to amplify an input signal power, over a range of frequencies, equal in amplitude or value and quality without an undesired deviation from a generally linear configuration. Characterizing the linear and non-linear behavior of power amplifiers involves defining the characteristics of power amplifiers, and presenting specific power amplifier requirements. A host of parameters are typically used to specify power amplifier performance. To test power amplifier performance, different test system architectures have been used. For example, amplifier testing is often performed under pulsed RF and pulsed bias conditions when testing unpackaged devices.
Moreover, newer technologies demand transmission of large amounts of data with only a small portion of the spectrum being used. This may be accomplished using sophisticated modulation techniques, leading to wide, dynamic signals that benefit from linear amplification. For instance, some modem wireless applications, such as based on the wideband code division multiple access (WCDMA) standard use non-constant envelope modulation techniques with a high peak-to-average ratio. To attain high data rates and spectral efficiency, these modem wireless communication standards employ non-constant envelope modulation techniques, such as quadrature phase shift keying (QPSK). The RF power amplifiers implemented in such systems are ‘backed off’ from saturation into a linear operating region to obtain a satisfactory linearity over the transmitter's dynamic range. Therefore, linearity being a critical issue, power amplifiers implemented in such applications are commonly operated at a backed off region from saturation. The non-linearity of a power amplifier can be attributed at least partially to gain compression and harmonic distortions resulting in imperfect reproduction of the amplified signal. This non-linearity may be characterized by various techniques depending upon specific modulation and application. Some of the widely used figures for quantifying linearity are a 1 dB compression point, third order inter-modulation distortion, third order intercept point (IP3), adjacent channel power ratio (ACPR), and error vector magnitude (EVM). A set of “standard” figures of merit that have been used to describe the behavior of amplifiers include parameters, such as a gain, third order intercept point (IP3), 1 dB compression point (P1dB) etc. All of these figures of merit are measured using quasi-static signals or in most cases even a constant wave (CW) RF signal without modulation contents. Referring to FIG. 3, for example, a typical characteristic of a radio frequency (RF) amplifier is depicted with the P1dB compression point based on a prior art figure of merit to describe the behavior of the RF amplifier.
In modern mobile communication networks, however, the transmission gets more and more complex using higher order modulation schemes, resulting in non-constant envelope signals. The linearity demands for power amplifiers increase rapidly by applying such signals. So the well-known parameters named before are no longer sufficient to describe the ability of an amplifier.
One parameter of non-constant envelope signals is a long time average power. With a peak-power to average-power ratio (PAR), a reasonable estimate of the nature of a signal may be derived. Because of this PAR, the average-power capability of an amplifier is, of course, less than the P1dB which is a significant measurement obtained with the CW-RF signals. To describe the power handling capability of a power amplifier, a term called “power backoff” is typically used. Because of the PAR, and additionally due to the nonlinearities in the amplifier, the average power of an amplified signal is normally well below the P1dB. The maximum average power still fulfilling the linearity specifications like error vector magnitude (EVM) or adjacent channel power ratio (ACPR) is now the upper limit for the amplifier. The power backoff now describes the ratio of the P1dB and this maximum average ratio. Assuming an ideal linear amplifier, the power backoff should be the PAR of a test signal.
However, one disadvantage of the power backoff based figure of merit is the very definition of the P1dB itself. That is, the power backoff based figure of merit is measured under conditions not comparable to that of the test signals used to determine the maximum average power. In fact, this figure of merit cannot be used to effectively compare the capabilities of two different amplifiers.
For example, a comparison of compression behavior of two different amplifiers is shown in FIG. 4. An amplifier 1 has a sharp compression behavior, so the P1dB is high and very close to the maximum possible output power. In contrast, an amplifier 2 has a softer compression behavior, with the P1dB being comparably lower than that of the amplifier 1. Both the amplifiers are assumed to exhibit the same gain for low input power. Assuming that both the amplifiers can deliver the same maximum average power the power backoff is different even the absolute maximum power Pmax is again the same. So in fact, the two amplifiers are comparable in their behavior but the figure of merit of power backoff presents a completely different and invalid view.
An additional significant drawback of the “standard” figures of merit mentioned earlier is that they are determined by using an unpulsed CW signal. A CW test signal is a sinusoidal signal with a PAR of zero. Thus, the CW test signal has substantially no relationship to the signal that the amplifier has to amplify during field or real world operation. Additionally, an unpulsed measurement at high output levels heats up the amplifier. This heating reduces the maximum output power, the P1dB and the gain of the amplifier. Accordingly, the maximum output power capability of the amplifier for the short peaks of a signal with a non-zero PAR cannot be accurately determined by using unpulsed CW test signals.
The present invention is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.